1. Field of the Invention
The present invention concerns a travelling-wave tube including a delay line contained in a sleeve. It concerns, more particularly, a coupling device between the delay line of the tube and external circuits for injection or for removal of microwave energy into or from the tube.
The invention is particularly suited to power travelling-wave tubes with wide band, the delay line of which has a helix structure. It can also be applied to tubes for which the delay line is derived from the helix structure, for example so-called "ring and bar" or "ring and loop" delay lines.
2. Description of the Prior Art
A travelling-wave tube or TWP generally comprises:
an electron gun which produces a long and narrow beam of electrons,
a delay line contained in a sleeve, where the interaction between the electron beam and a microwave takes place. This delay line is provided with a device for injection and a device for removal of microwave energy,
a focusing device using permanent magnets placed around the delay line throughout its length. This focusing device creates a magnetic induction in the zone of interaction between the electron beam and the microwave,
a collector which can collect the electron beam.
The injection of microwave energy, at the input of the delay line, is generally done by means of a coaxial connector, the injected energy level being low.
The amplified energy can be removed, at the output of the delay line, either by means of a ridged waveguide or by means of a coaxial element. This depends on the energy level present at the output of the delay line.
In all cases, the external surface for the injection and removal of microwave energy are specially designed to ensure the vacuum tightness of the interior of the travelling-wave tube.
A coupling device, between the delay line of a travelling-wave tube and the external microwave circuits is known, for example, from the French patent No. 2 485 801-B filed on 27th June 1980. In this patent, the delay line is a helix. At least one of its ends is connected to an external circuit. This external circuit has a transmission line possessing an internal conductive core. The connection is made by means of a coupling pin. The coupling pin is solidly joined to one side of the internal conductive core of the transmission line. On the other side, the coupling pin is brazed to the end of the helix. In this patent, the transmission line is either a ridged waveguide, the conductive internal core being the ridge, or it is a coaxial line, the conductive internal core being the central conductor of this line.
When assembling TWTs, it is seen that the point where the coupling pin should be brazed, at the end of the helix, is not easily reached. Furthermore, the dimensions of the helix and of the coupling pin are very small. The measurements to be made to ascertain that the TWT conforms to requirements can be done only after the helix/coupling pin junction has been made. Since the connection between these two parts is a brazing, it cannot be disconnected. This means that, if there is a defect in one of the parts forming either the delay line or the focusing device or the external microwave circuits, the entire unit will be rejected.
The present invention is aimed at overcoming these drawbacks. It proposes to establish mechanical independence between the delay line and the external circuits for the injection or removal of the microwave energy into or from a travelling-wave tube.
For this purpose, the coupling pin, which, in the prior art, was joined solidly to the conductive internal core of the transmission line, will be kept solidly joined to a conductive part. The conductive part is fixed to the sleeve surrounding the delay line. The coupling pin is solidly joined to the conductive part and is fixed to the end of the delay line. It is through the external surface of the part that the electromagnetic coupling between the delay line and the conductive core of the transmission line is made.
During the assembly of TWTs using the device according to the invention, it is possible to identify defects, if any, in the elements forming either the delay line or the focusing device or the external circuits, before the assembly is finalized. Measurements can be made between the delay line and the external circuits without the assembly of these parts having been finalized. The defective elements alone will be rejected. The cost of fabrication of the TWTs used in this coupling device will be reduced.